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Etymologies

After a cartoon character created in 1942 by George Baker (1915-1975).

(American Heritage® Dictionary of the English Language, Fourth Edition)

US 1920s. Popularized by Sad Sack, a cartoon character and eponymous comic strip published originally June 1942 in Yank, the Army Weekly, a US Army publication for soldiers, and later syndicated in the US 1940s and 1950s. Presumably from vulgar “sad sack of shit”; Cartoonist Sgt. George Baker said he took from a “longer phrase, of a derogatory nature”. The term originally referred to a well-meaning but inept soldier. (Wiktionary)